The sea constitutes a dynamic environment that imparts dynamic loads on hoisting cables unlike those imparted in a land-based operation. In particular, the at-sea environment can significantly decrease the safety factor of the lifting line. This is particularly true when the sea generated effects of roll, pitch and yaw cause the line of non-motion compensated lifting equipment to go slack for an instant, followed by a sudden dynamic loading. This situation, a condition called "snap loading", is a major cause of lift line failure because the stress in the line can be many times that due to the static weight of the object being lifted.
In addition to lifting a load with a crane where the load and crane are mounted on separate platforms moving relative to each other, it may be necessary to lift objects in or floating on the sea. Further, launch and recovery operations of boats, sea planes or other items may be needed. In recovery operations the objects to be lifted may be flooded with water further temporarily raising the load levels to the line and crane.
Launch and recovery operations are presently accomplished using a host ship's crane, connected directly to the object to be lifted. High loads and jerks (snap loading) on the lifting line due to the relative movement between the crane boom and the object can cause damage to the ship's crane, lift line, and/or the lifted object.
One method of reducing this snap loading is to install a motion compensating device in the rope between the crane and the object to be lifted. This motion compensating device will limit the maximum tension by taking up and letting out of line. The motion compensating device assists by placing "give" into the system. The motion compensating device is essentially a large air spring which accommodates crane boom tip movement without snap loading. This is because the motion compensating device will not allow the load lift line to go slack during operation. The load line is taken up by the motion compensating device as the boom moves down. Conversely as the boom moves up, the motion compensating device can give out load line as its hydraulic cylinder compresses the gas in the accumulator like a spring. This compressed gas can then extend the ram and take up load line as load is decreasing. An added feature of using a motion compensating device is that since the lift system is now soft due to the takeup and give out of load line in the motion compensating device, the object to be lifted will not be moved nearly so vigorously in the water by the motion from the crane boom with respect to the water surface. Thus, use of a motion compensating device will provide a much steadier platform of the lifted object for access by swimmers in recovery operations.
Because the motion compensating device minimizes dynamic loads on the lift line, design factors of safety can be reduced somewhat from conventional practice. Prior to the development of motion compensating devices, designers were forced to utilize very high factors of safety in developing lifting equipment for use at sea to assure that a lift line would not part should it undergo a series of snap loading cycles. For example, conventional design practice would dictate applying a six to one safety factor on the fully flooded weight of a recovery operation at sea in designing lift equipment. However, with the use of a properly designed motion compensating device, not only is the possibility of snap loading virtually eliminated, the motion compensating device assures that the object lifted from the water is lifted slowly enough that the lift system never even sees a fully flooded object.
The possibility of snap loading is eliminated through two means, first by preventing the lift line from going slack and second by lengthening the acceleration time and therefore lowering the peak loads in the line. As a typical case consider that the object is supported by the motion compensating device and the boom tip moves up suddenly. As the boom tip moves upward, it applies a slightly greater load to the object through the motion compensating device. The motion compensating device seeing this greater load reacts by compressing the gas in the accumulator and paying out wire rope to the load. The object in turn seeing this greater load will start to accelerate to the boom tip velocity. This acceleration to boom tip velocity will take time and the compensator will have compressed some distance during this time. The amount of time or compression distance that this acceleration takes can be controlled by changing the size of the accumulator of the motion compensating device. Ultimately, the difference between using a motion compensating device versus not using a motion compensating device during lifting, deployment, launch and recovery operations through the air/water interface, is that the acceleration time on the payload is an order of magnitude different in favor of using the motion compensating device.
An object of the invention is to provide a portable balanced motion compensating apparatus for use in raising and lowering loads in a marine environment.
Another object of the present invention is to provide a portable balanced motion compensating apparatus small enough and light enough to suspend from existing cranes.
Another object of the invention is to provide an apparatus that eliminates snap loading caused by crane boom and load relative movement in a marine environment.
A further object of the invention is to provide an apparatus that would be simple to operate.
Various other objects, features and advantages of the invention will be apparent in the following drawings and description of the invention.